Ultrasonic imaging from within the body of a patient has been used for some time, particularly in the treatment of heart disease. While alternative methods of sensing the condition of diseased vasculature exist, such as the injection and monitoring of radiopaque dyes, ultrasound is the most promising technology for accurately viewing the interior of a body in real time and in a non-destructive manner.
Such imaging techniques are particularly useful in connection with an angioplasty device that removes a built-up deposit within a lumen. Successful removal depends upon accurately locating the deposit in relation to the device. It is desirable that the resolution of a visualization technique be at least commensurate with the degree of resolution of the ablation device. The present invention has particular utility with high resolution ablation devices. An example of such a device is described in U.S. Pat. Nos. 5,626,576 and 5,454,809, commonly owned with the present invention. In such a device, radio frequency current is selectively deployed around the circumference of a lumen such as a coronary artery, depending upon the position of the occlusive material. The imaging used with such a device should enable the user to determine the circumferential position of the deposit. In general, prior art ablation techniques have not required the visualization resolution provided by the present invention.
In an in vivo ultrasound imaging system, an array of piezoelectric transducer elements residing on a catheter are introduced into a body. The elements are excited at ultrasound frequencies to transmit acoustical waves, and receive echos as the acoustical waves reflect from the surrounding material. The echos provide electrical signals which are processed to form the ultrasound image.
Previous in vivo ultrasound images have included a number of limitations. The simplest systems use a mechanical scanning system. A flexible drive cable rotates a single element to scan a cross sectional image of a lumen. Problems associated with mechanical systems include jitter, and the limitation of fixed transmit and receive focus restricts clarity and resolution of an image away from the focal point.
Current ultrasound systems utilize sampled phased (PS) arrays, wherein one wire is multiplexed to any one of the elements of an ultrasound array. A first element is individually pulsed, and then the same element receives the echos. A next element is then pulsed and received, and the process is repeated for each array element. A problem with this technique is that cross-product terms are ignored. A cross-product term is where, for example, a first element is pulsed and a second element receives the pulse. This lack of cross-product terms caused grating lobes and increased sidelobe levels. See O'Donnel et al., "Experimental Studies on an Efficient Catheter Array Imaging System" (Ultrasonic Imaging 17, pp. 83-94, 1995). While some modern systems do include cross-product terms (see U.S. Pat. No. 5,590,659 (Hamilton et al., 1997), they are believed to involve a relatively high number of processing steps to do so. This is because the cross products are determined through mathematical reconstructions after transmitting with a single element and receiving with another single element.
Additional problems associated with PS systems are acoustic pressure limitation and ring down artifact. Regarding the acoustic pressure limitation, the PS system cannot receive greater pressure than a single element can provide. Element SNR (signal-to-noise ratio) is typically less than 0 dB. A compensatory technique for such low SNR is averaging multiple independent actuations for each element. While such an averaging technique reduces noise, it can result in noticeable image smearing. The main component of ring down is when the transmit and receive element are the same. When such an element is first switched to receive, it is still "ringing" from transmit. One such PS ultrasound system is described in U.S. Pat. No. 4,917,090 of Proudian et al. In that system a 1.times.16 multiplexer is provided; that is, a multiplexer selects one transducer element from an array of 16 elements in both transmit and receive modes. To form a useful image from a limited sample, a synthetic aperture focusing technique is used. While that may provide acceptable image quality is some applications, the limited partial sampling is not believed to provide sufficient image quality required for the successful application of advanced ablation techniques such as described above.
It can be appreciated that an in vivo ultrasound visualization system providing improved image quality will allow for more successful treatment of occluded lumens. A key component of such a system is an multiplexer/preamplifier circuit as provided in the present invention. Such a circuit must be able to be physically inserted within a lumen, which imposes significant design constraints. The circuit described herein has particular, although not exclusive, application with a transducer array probe such as described in co-pending application "Ultrasound Transducer Array Probe for Intraluminal Imaging Catheter" filed under atty. docket no. 010848-0017. While the present invention may be described in the context of a coronary artery and atherosclerosis, the application of the invention is not so limited and may be used in the imaging of any lumen within a body, or, conceivably, any other enclosed area.
All documents referred to herein are hereby incorporated by reference to the extent they contain information necessary or helpful to an understanding of the present invention.